Following accidental exposure of humans to irradiation, the dose to different parts of the body is often highly heterogeneous, and regions of the skin may receive substantially greater doses than other parts of the body. The goal of this project is to develop biological approaches that can be used to assess dose to the skin, with a particular emphasis on DNA damage in cells of the dermis and epidermis. Our previous work has demonstrated that it is possible to detect micronuclei (MN) in skin fibroblasts from both rats and humans taken weeks after irradiation. Thus, our hypothesis is that, following exposures due to accidents or terrorist-instigated incidents, it will be possible to estimate skin dose using a small punch (3 mm) biopsy that could be taken within a number of days or weeks after radiation exposure. The project will have four specific aims. The first aim builds on our previous studies and will refine our existing techniques to establish optimal methods to measure MN obtained from dermal fibroblasts using the smallest possible biopsy, working initially with mice and then with human skin. The second specific aim will be to investigate the potential for predicting the dose received by the skin, using measurements of radiation-induced foci (RIF) of proteins in the cell nuclei performed directly on sections obtained from skin biopsies. Again, the work will be initiated in skin from various different strains of mice (wild-type and with known genetic defects that influence radiosensitivity e.g. SCID and ATM knock-outs) and will be followed up in studies of human skin. The third specific aim will extend these techniques to examine exposure to high LET radiation (thermal neutrons). The focus of these studies will be to obtain information on RBE values relevant to accidental exposure using these techniques. The final specific aim will integrate these studies in skin with techniques being developed in other projects in the Center in animals given whole body irradiation or partial body irradiation, which simulate the conditions expected following exposure of humans in an accidental or terrorist-instigated radiation exposure;we will use the derived endpoints to examine the ability of mitigating agents developed in the program to mitigate against skin injury.